NASA - OK site, but overrated and hard to navigate. There are so many sub pages within the site such as the JPL and other mission sites.

SETI@home - A scientific experiment that harnesses the power of hundreds of thousands of Internet-connected computers in the Search for ExtraTerrestrial Intelligence). You can participate by running a free program that downloads and analyzes radio telescope data. There's a small but captivating possibility that your computer will detect the faint murmur of a civilization beyond Earth.

The Ultimate Space Place - Space history, memorabilia, missions, and more. Easy to navigate and loaded with useful information. Includes all space missions from Mercury to the space shuttle.

Astronomy Magazine - The most popular astronomy magazine in the world sales wise. Good content, but lack of information.. more ads than pictures.

Forces of a BlueMoon - Only site on bluemoons I know of. Mostly personal beliefs and other spiritual things related to blue moons.

Astrophotography - Friend of mine who takes his own pictures with his scope. He takes good photos and shows off his scope with pictures of it and the camera he uses.

Kitahara PhotoGallery - Great photos of distance galaxies taken with a advance (an expensive) telescope. Also contains a great adjuster to set you monitor to the correct contrast and brightness.

Astronomy Now - Excellent magazine with the best content and images around. Based in the UK shipping and handling is a pain which is why many choose "Astronomy Magazine" instead.

Astronomy in your face - Exactly what the title says. Pictures, information, links, and more. Includes a "sky journal" and news updates.

Stephen Hawking's Universe - The best TV program and book ever. You will learn more with this book and movie than you will reading any other video/book set.

Sidewalk Astronomers - This guy has some amazing ideas and theories, but good luck understanding them. Not for the beginner or intermediate astronomer.

M.I.T. Space Plasma Group - Another site for only experienced users.

Astronomy and Cosmology - Decent amount of information on cosmology and definitions of cosmology terms.